1. Field of the Invention
The present invention pertains to electronic imaging, in particular to the generation of interpolated color images.
2. Description of Related Art
Currently, because of the advantages of having simple structure and being easily operated, the single-chip color sensor is in wide spread use in color electronic cameras to capture images. Generally, a single-chip color sensor has a plurality of pixels arranged in same form of two-dimensional array, and the sensor is covered by a color filter. The color filter has three primary colors including red (denoted by “R”), blue (denoted by. “B”) and green (denoted by “G”). When capturing images, each pixel can only sense the signal value of a corresponding primary color, while missing the values of the other two primary colors. Therefore, an image processing method must be further utilized to determine the values of the two missing colors for the pixel. Such an image processing method is related to the arrangement of the three primary colors in the color filter. One of the most commonly used color arrangements, known as the Bayer pattern, is described in U.S. Pat. No. 3,971,065. The Bayer pattern is shown in FIG. 6, wherein each color covers a pixel of the sensor. In particular, the R and B colors are interspersed among a checkerboard pattern of G colors.
As well known to those skilled in the art, an interpolation method is often provided to increase the number of image sampling points in processing images. This interpolation method can also be used to determine the interpolated values of the missing colors for each pixel in an image system utilizing the single-chip color sensor, so as to compensate for those missing colors, thereby producing a full color image and maintaining the resolution. However, as one can expect, if the interpolated values determined by the interpolation method are not close to the actual ones, a known false color problem will be encountered. Moreover, because of limitation in the manufacturing process, the pixels in the sensor are rarely identical. In particular, with reference to FIG. 5, the G pixels in line 51 are alternately arranged with the B pixels, while the G pixels in line 52 are alternately arranged with the R pixels. Therefore, the G pixels in the line 51 and 52 are usually not identical. The use of a sensor with such different G pixels to capture images may encounter a check pattern noise problem.
Several issued patents are provided to produce full color images by interpolation techniques. In U.S. Pat. No. 4,642,678, a signal processing method is disclosed for producing interpolated chrominance (i.e. R or B color) values in a sampled color image signal. In the method, hue values at neighboring chrominance component sample locations are produced as a function of a luminance (i.e. G color) value and the chrominance value at the neighboring locations. A signal representing an interpolated hue value is produced as a function of neighboring hue values. A signal representing an interpolated chrominance value is produced as a function of the interpolated hue value and a luminance value at the interpolated location.
A problem with the '678 patent is that it is prone to having a zigzag pattern on the edge of an image. This problem can be treated through use of more sophisticated interpolation techniques, such as described in U.S. Pat. No. 4,630,307, which first determines the geometric shapes, including edges, stripes and corners, contained in the image, and then applies different interpolation routines to different shapes. Because of the sophisticated procedure, this method performs the interpolation process at considerable cost and complexity in processing capability.
In U.S. Pat. No. 5,373,322 entitled “Apparatus and Method for Adaptively Interpolating a Full Color Image Utilizing Chrominance Gradients”, adaptive interpolation is performed by apparatus operating upon a digitized image signal obtained from an image sensor. A digital processor obtains gradient values from the differences between chrominance values in vertical and horizontal image directions. The gradient values are compared with each other in order to select one of the directions as the preferred orientation for the interpolation of additional luminance values. The interpolation is then performed upon values selected to agree with the preferred orientation. The interpolation method of the '322 patent is able to enhance the image resolution and reduce the false color effect. However, the method requires a relative large memory space to store the sampled neighboring points for determining the missing color values of a pixel. In particular, a memory space capable of storing at least four sampling image lines must be provided to execute the interpolation process. If a single-chip color sensor has a resolution of 640×480, a total memory space of 640×4×8 bits must be provided.
Therefore, it is known that large memory space and complicated interpolation processes must be employed to perform conventional interpolation methods, which further results in complicated hardware structure and high power consumption. Furthermore, the check pattern noise problem is not removed in using the conventional interpolation methods. Accordingly, it is desired to have a novel interpolation method to obviate and/or mitigate the aforementioned problems.